The present invention generally relates to photodetectors, and more particularly to a photodetector which has a photodetector part and a signal processing part provided on independent semiconductor substrates.
An infrared sensor (infrared image pickup element) has a photodetector part and a signal processing part provided on independent semiconductor substrates, and the photodetector part and the signal processing part are connected by bumps or the like. The infrared sensor is applied to apparatuses which detect and track an object by sensing the infrared ray which is emitted from the object. The infrared sensor which is applied to such apparatuses must be able to detect a fine object at a far distance.
FIG. 1 generally shows an example of a conventional infrared photodetector. In FIG. 1, (A) shows a plan view and (B) shows a cross sectional view along a line A--A in (A). This infrared photodetector includes a photodetector part 1 and a signal processing part 2 which are connected via electrodes 3.
The photodetector part 1 includes a p-type semiconductor substrate 1a, and a plurality of n-type semiconductor regions 1b which are formed in the p-type semiconductor substrate 1a by employing a diffusing technique or the like. For example, the p-type semiconductor substrate 1a is made of Hg-Cd-Te. The n-type semiconductor regions 1b have a circular shape in the plan view, and are arranged in a 3.times.3 matrix arrangement, for example. A pn junction 4 is formed between the p-type semiconductor substrate 1a and each n-type semiconductor region 1b.
The signal processing part 2 is formed by a Si charge coupled device (CCD) or the like. The signal processing part 2 is connected to the n-type semiconductor regions 1b of the photodetector part 1 via the electrodes 3, and processes signals received from the photodetector part 1. The signal is output from the photodetector part 1 when the p-type semiconductor substrate 1a receives infrared ray and a photoelectric conversion occurs via the pn junction 4. For example, the electrodes 3 are formed by bumps.
FIG. 2 is a diagram for explaining the operation of the infrared photodetector shown in FIG. 1. In FIG. 2, (A) shows a cross sectional view of the p-type substrate 1, and (B) shows the sensitivity versus position characteristic of one n-type semiconductor region 1b in correspondence with (A).
As shown in (A) of FIG. 2, electrons are generated in the p-type semiconductor substrate 1 when the infrared ray is irradiated on the photodetector part 1. Because the pn junctions 4 are reverse biased, the generated electrons are collected at the n-type semiconductor regions 1b and move to the signal processing part 2 via the electrodes 3.
Each n-type semiconductor region 1b has the sensitivity shown in (B) of FIG. 2. As shown, the sensitivity is a maximum at the center of the n-type semiconductor region 1b and decreases towards the periphery of the n-type semiconductor region 1b.
On the other hand, the electrodes generated in the p-type semiconductor substrate 1a disappear after moving a diffusion length Le which is an average distance movable by each electron. Accordingly, in mid or far infrared photodectors for detecting the wavelengths of 2 to 15 .mu.m, the diffusion length Le becomes 10 .mu.m or greater, for example, and the diffusion length Le can no longer be neglected compared to the diameter d of the n-type semiconductor region 1b and the pitch P at which the n-type semiconductor regions 1b are arranged. For this reason, in such mid or far infrared photodetectors, the electron which is generated at a distance Le from the pn junction 4 is also collected by the n-type semiconductor region 1b as shown in FIG. 3. In FIG. 3, (A) shows the cross section of the photodetector part 1 and (B) shows the plan view of the photodetector part 1. In the case shown in FIG. 3, an effective photosensitive area of the n-type semiconductor region 1b becomes a distance L (.apprxeq.Le) greater in radius compared to the area of the pn junction 4, as indicated by a dotted line. This area indicated by the dotted line substantially corresponds to the size of one pixel.
FIG. 4 is a diagram for explaining examples of the light receiving range of the conventional photodetector. In FIG. 4, each region surrounded by a dotted line indicates one n-type semiconductor region 1b, and each region surrounded by a solid line indicates the effective photosensitive area of one n-type semiconductor region 1b. As shown in FIG. 4, the adjacent n-type semiconductor regions 1b are arranged so that the effective photosensitive areas thereof do not overlap, in order to prevent crosstalk between the adjacent n-type semiconductor regions 1b.
In FIG. 4, (A) shows a case where the n-type semiconductor regions 1b indicated by the dotted line have a circular shape in the plan view. In this case, each effective photosensitive area 5 of the n-type semiconductor region 1b also has a circular shape in the plan view as indicated by the solid line.
In FIG. 4, (B) shows a case where the n-type semiconductor regions 1b indicated by the dotted line have a square shape in the plan view. In this case, each effective photosensitive area 5 of the n-type semiconductor region 1b has a generally square shape with rounded corners in the plan view as indicated by the solid line.
In FIG. 4, (C) shows a case where the n-type semiconductor region 1b indicated by the dotted line have a square shape in the plan view. In this case, each effective photosensitive area 5 of the n-type semiconductor region 1b has a generally square shape with rounded corners in the plan view as indicated by the solid line. But unlike the case shown in FIG. 4 (B), the positions of the n-type semiconductor regions 1b in one horizontal row are shifted in the horizontal direction with respect to the positions of the n-type semiconductor regions 1b in the next horizontal row. In other words, the positions of the n-type semiconductor regions 1b are not aligned vertically as in the case shown in FIG. 4 (B).
In FIG. 4, a blind area 6 is formed between the adjacent effective photosensitive areas 5. The shape of the effective photosensitive areas 5 shown in (B) enables the blind area 6 to be reduced compared to the case shown in (A). In the case shown in (C), each blind area 6 is smaller than the blind area 6 shown in (B), but the number of the blind areas 6 is larger in (C) than in (B), and the total of the blind areas 6 in (C) is not reduced compared to the total of the blind areas 6 in (B).
Therefore, the conventional infrared photodetector suffered from the following problems. First, if the n-type semiconductor regions 1b are arranged so that the effective photosensitive areas 5 do not overlap, the blind areas 6 are inevitably formed and the infrared ray cannot be detected at the blind areas 6. Second, if the areas of the n-type semiconductor regions 1b are increased so as to reduce the blind areas 6, the effective photosensitive areas 5 overlap and the crosstalk between the adjacent n-type semiconductor regions 1b increases.
On the other hand, in the photodetectors which detect visible rays, the diffusion length Le is negligibly small, and the shape of the n-type semiconductor region 1b substantially corresponds to the effective photosensitive area. The visible ray photodetector has a monolithic structure, and the pn junction, signal transmission lines, switching gates and the like are all formed on a single Si substrate, for example. Because the parts of the Si substrate where the signal transmission lines, the switching gates and the like are provided become blind areas, measures were conventionally taken to reduce the size of the transmission lines, switching gates and the like in order to increase the photodiode area. However, such conventional measures taken in the visible ray photodetector cannot be applied to the infrared photodetector, that is, do not help solve the problems described above.